Radiation-induced heart disease (RIHD) is a potentially severe side effect of radiotherapy of thoracic and chest wall tumors. Despite advances in radiation delivery and treatment planning technology, cardiac radiation toxicity remains a key consideration in thoracic radiotherapy. Hence, research is urgently needed to elucidate biological mechanisms and identify potential targets for intervention. Our previous pre-clinical studies in a mast cell-deficient rat model have shown that, contrary to what had been the prevailing assumption, mast cells play a predominantly protective role in RIHD. The overall goal of this project is to uncover the role of three mast cell targets in RIHD. For this purpose, genetic animal models and pharmacological interventions will be used to define the involvement of 1) Proteinase-Activated Receptor (PAR)-2, the receptor that mediates cellular effects of mast cell proteinases; 2) bradykinin, a cardioprotective hormone that is formed by mast cell proteinases; and 3) the cardioprotective sensory neuropeptide calcitonin gene-related peptide (CGRP), as part of the neuroimmune interactions that are known to alter cardiac function and disease. Functional, structural, cellular and molecular manifestations of RIHD will be measured after local heart irradiation. These in vivo experiments will be the first to provide mechanistic insight into the protective effects of mast cells in RIHD, providing critical pre-clinical data as a basis for future development of intervention strategies, thereby enhancing the efficacy and safety of thoracic radiotherapy.